The following is an example of drawing curves with multiple points on an input surface and extruding those curves. The sample input file used in the example is below.

surface8.3dm

import processing.opengl.*;
import igeo.*;

size(480, 360, IG.GL);

IG.open("surface8.3dm");
ISurface[] surfaces = IG.surfaces();

int unum = 10, vnum = 30;
double uinc = 1.0/unum, vinc = 1.0/vnum;

for (ISurface surf : surfaces) {
  for (int i=0; i < unum; i++) {
    IVec[] pts = new IVec[vnum+1];

    for (int j=0; j <= vnum; j++) {
      if (j%2==0) {
        pts[j] = surf.pt(i*uinc, j*vinc);
      }
      else {
        pts[j] = surf.pt((i+1)*uinc, j*vinc);
      }
    }
    new ICurve(pts).clr(0);
  }
  surf.del();
}

Then amount of undulation is controlled by image mapping. The degree of the curve is also changed to create curvature. The input bitmap used in the example is this.

import processing.opengl.*;
import igeo.*;

size(480, 360, IG.GL);

IG.open("surface8.3dm");
ISurface[] surfaces = IG.surfaces();

int unum = 20, vnum = 60;
double uinc = 1.0/unum, vinc = 1.0/vnum;

IImageMap map = new IImageMap("map3.jpg");

for (ISurface surf : surfaces) {
  for (int i=0; i < unum; i++) {
    IVec[] pts = new IVec[vnum+1];

    for (int j=0; j <= vnum; j++) {
      if (j%2==0) {
        pts[j] = surf.pt(i*uinc, j*vinc);
      }
      else {
        // bitmap value is sampled. note that u and v is swapped to match with u-v of the input surface.
        double val = map.get( j*vinc, i*uinc );

        pts[j] = surf.pt((i+val)*uinc, j*vinc);
      }
    }
    new ICurve(pts, 2).clr(0);
  }
  surf.del();
}

Then the curves are extruded in the y-axis direction.

import processing.opengl.*;
import igeo.*;

size(480, 360, IG.GL);

IG.open("surface8.3dm");
ISurface[] surfaces = IG.surfaces();

int unum = 20, vnum = 60;
double uinc = 1.0/unum, vinc = 1.0/vnum;

IImageMap map = new IImageMap("map3.jpg");

for (ISurface surf : surfaces) {
  for (int i=0; i < unum; i++) {
    IVec[] pts = new IVec[vnum+1];

    for (int j=0; j <= vnum; j++) {
      if (j%2==0) {
        pts[j] = surf.pt(i*uinc, j*vinc);
      }
      else {
        // bitmap value is sampled. note that u and v is swapped to match with u-v of the input surface.
        double val = map.get( j*vinc, i*uinc );

        pts[j] = surf.pt((i+val)*uinc, j*vinc);
      }
    }
    ICurve crv = new ICurve(pts, 2).clr(0);
    IVec extrudeDir = new IVec(0, -2, 0);
    IG.extrude(crv, extrudeDir).clr(0,0,i*uinc,0.5);
  }
  surf.del();
}

Then the depth of extrusion is randomized and pipe geometry is added on the edge of the extruded surface.

import processing.opengl.*;
import igeo.*;

size(480, 360, IG.GL);

IG.open("surface8.3dm");
ISurface[] surfaces = IG.surfaces();

int unum = 20, vnum = 60;
double uinc = 1.0/unum, vinc = 1.0/vnum;

IImageMap map = new IImageMap("map3.jpg");

for (ISurface surf : surfaces) {
  for (int i=0; i < unum; i++) {
    IVec[] pts = new IVec[vnum+1];

    for (int j=0; j <= vnum; j++) {
      if (j%2==0) {
        pts[j] = surf.pt(i*uinc, j*vinc);
      }
      else {
        // bitmap value is sampled. note that u and v is swapped to match with u-v of the input surface.
        double val = map.get( j*vinc, i*uinc );
        pts[j] = surf.pt((i+val)*uinc, j*vinc);
      }
    }
    ICurve crv = new ICurve(pts, 2).clr(0);
    IVec extrudeDir = new IVec(0, IRandom.get(-3, -1), 0);
    IG.extrude(crv, extrudeDir).clr(0,0,i*uinc,0.5);

    ICurve crv2 = crv.cp(extrudeDir);
    IG.pipe(crv2, 0.1).clr(0.2);
  }
  surf.del();
}